First measurement of jet mass in Pb–Pb and p–Pb collisions at the LHC

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Physics Letters B

www.elsevier.com/locate/physletb

First measurement of jet mass in Pb–Pb and p–Pb collisions at the LHC

ALICE Collaboration

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received26April2017

Receivedinrevisedform20October2017 Accepted21November2017

Availableonline23November2017 Editor: L.Rolandi

ThisletterpresentsthefirstmeasurementofjetmassinPb–Pb andp–Pb collisionsat√_s

NN=².76 TeV and√_s

NN=⁵.02 TeV,respectively.Boththejetenergyandthejetmassareexpectedtobesensitiveto jetquenchinginthehotQuantumChromodynamics(QCD)mattercreatedinnuclearcollisionsatcollider energies. Jets are reconstructedfromcharged particles usingthe anti-kT jet algorithmand resolution parameter R=⁰.4.Thejetsaremeasuredinthepseudorapidityrange|ηjet|<0.5 andinthreeintervals oftransversemomentumbetween60GeV/cand120GeV/c.Themeasurementofthejetmassincentral Pb–Pb collisions is compared to the jet mass as measured in p–Pb reference collisions, to vacuum event generators,and to models includingjet quenching. It is observed that the jet mass incentral Pb–Pb collisionsisconsistentwithinuncertaintieswithp–Pb referencemeasurements.Furthermore,the measuredjet massinPb–Pb collisionsisnot reproducedbythe quenchingmodelsconsidered inthis letterandisfoundtobeconsistentwithPYTHIAexpectationswithinsystematicuncertainties.

©^{2017TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

ThisletterpresentsthefirstmeasurementofjetmassinPb–Pb and p–Pb collisions at √

sNN=².76 TeV and √

sNN=⁵.02 TeV, respectively. Both the jet energy and the jet mass are expected to be sensitive to jet quenching in the hot Quantum Chromody- namics(QCD) matter, theQuark–Gluon–Plasma (QGP), createdin ultra-relativisticnuclear collisions.Scattering processeswithlarge momentumtransfer, Q²,betweenthequarksandthegluons(par- tons)constituentsofcollidingnucleonsoccurearlyinthecollision (at a time <1fm/c). Outgoing partons carry a net color charge and evolve from high to low virtuality producing parton show- ers,whicheventuallyhadronizeintocollimatedspraysofparticles, calledjets.Interactionsofthe outgoingpartonswiththe hot and denseQGPcreatedinheavy-ioncollisionsmaymodifytheangular andmomentumdistributionsofhadronicjetfragmentsrelativeto jetsfragmentinginvacuum.Thisprocess,knownasjetquenching, canbeusedtoprobethepropertiesofthehotQCDmedium[1–4].

Jet quenching has been investigated at the Relativistic Heavy IonCollider (RHIC)[5–9] and atthe LargeHadronCollider (LHC) [10–20] via measurements of high-pT hadrons and fully recon- structed jetsinnucleus–nucleus (AA)collisions and pp (vacuum) collisions. These measurements have shown a suppression of hadron and jet yields in AAcollisions and modest modifications ofthelongitudinalfragment distribution andtheradial profileof jets relative to jets produced in pp collisions within the typical

E-mailaddress:alice-publications@cern.ch.

jet cone of 0.3–0.4 at the LHC. The jet mass is sensitive to the initial virtuality of the parton at the origin of the shower [21].

Energy-momentumexchangewiththehotQCDmediummaytem- porarilyincreasethevirtualityofthepropagating partons,leading toalargergluonradiationprobability[22–25].Thiswouldresultin abroadeningofthejetprofileandanincreaseofthejetmass,ifa significantamountoftheradiatedgluonsarecapturedwithinthe jetconeusedforreconstruction.However,thevirtualityincreaseis temporaryanditisexpectedthattheleadingpartontraversinghot QCD matterexperiencessubstantial virtuality(or mass)depletion alongwithenergyloss[21].

Thejetmassofinclusivejetsandofjetsindijeteventshasbeen previouslymeasuredinhigh-energyppcollisionsat√

s=^{7 TeV at} the LHC [26,27].Perturbative QCD predictions using higher-order matrix-elements for parton production combined with a Monte Carlo (MC) parton shower were found to be in good agreement withthedata.Thecommonlyusedleading-ordereventgenerators withfullshower evolution,PYTHIA[28,29] andHERWIG[30],re- producethejet massdistributioninpp collisionsreasonablywell inthe p_T region200–600 GeV/c previouslystudied,howeverthey consistently under- and over-predict the data, respectively, by a slightamount.

In this letter, measurements of the charged-jet mass are re- ported.Chargedjetsarejetsclusteredusingonlychargedparticles, reconstructed in the ALICE tracking system, opposed to full jets, reconstructed with both charged and neutral particles. The four momentum of the jet is defined as the sum of constituent four momenta.Thejetmassiscalculatedfromthejetfour-momentum, https://doi.org/10.1016/j.physletb.2017.11.044

0370-2693/©^{2017TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

M

=

E²

−

^p2_T

−

^p2_z

,

(1)

whereE isthejetenergy,pT thetransverseand pzthelongitudi- nalmomentumofthejet.

The measurement is performed in Pb–Pb and p–Pb collisions and in three intervals of jet transverse momentum. Data-driven jet-by-jetbackgroundsubtractionschemesareusedtocorrectthe jet mass for the contribution of the Pb–Pb underlying event. In contrast, the p–Pb background is included in the response ma- trix andcorrected for in the unfolding, asdiscussed indetail in Sec.3.1and3.2.Thedataarecomparedatdetectorleveltoasim- ulated reference without jet quenching effects. Furthermore, the measurementis correctedto particlelevel viaa two-dimensional unfoldingtechnique, accountingfor theremaining effectof back- ground fluctuations and detector effects. The fully corrected jet massdistributionincentralPb–Pb collisionsiscomparedtomod- elsandtothejetmassdistributionmeasuredinp–Pb collisions.

2. Datasample

The Pb–Pb collision data were recorded during the 2011LHC Pb–Pb runat √

s_NN=².76 TeV. Thisanalysisused minimum-bias (MB) events,selectedonline by requiringa signal inthe forward V0 detectors,two arrays of scintillator tiles covering the full az- imuthwithin 2.8<

η

<5.1 (V0A)and−³.7<

η

<−¹.7 (V0C).An onlinecentralitytriggerselectedthe10%most-centralPb–Pb colli- sionsusingthecentralitydeterminationasdescribedin[31],with 100%efficiencyforthe0–8%centralityinterval,and60%efficiency forthe8–10% interval. Thenumber ofPb–Pb eventsusedin this analysis,aftertheeventselectiondescribedbelow,is17 millionin the0–10%centralityinterval.

CollisionsofprotonandleadbeamswereprovidedbytheLHC inthefirstmonthsof2013.Thebeamenergieswere4TeVforthe protonbeamand1.58TeVper nucleonfortheleadbeam, result- ing in collisions ata center-of-massenergy of √

sNN=⁵.02 TeV.

Thenucleon–nucleoncenter-of-masssystemmovesrelativetothe laboratoryframe withrapidity 0.465 in the directionofthe pro- ton beam [32]. In the following,

η

refers to the pseudorapidity in the laboratory frame. The V0 detectors were used for online minimum bias event triggering and offline event selection. The minimumbiastriggerrequiredasignalfromachargedparticlein boththeV0AandtheV0C.The totalintegratedluminosityofthe minimumbias eventsample is 37μb⁻¹. Inaddition, eventstrig- geredbyanonlinejet triggerusingtheelectromagneticcalorime- ter (EMCal) [33,34] were used. The online jet patch covered an areaofapproximately0.2 sr andrequiredanintegratedpatchen- ergyofatleast20 GeV.Thetransversemomentumdistributionsof charged jetsin the triggered sample was compared to the min- imum bias one, showing that the trigger was fully efficient for p_{T,ch jet}^{60 GeV/c.Theminimumbiasandtriggeredsamplewere} usedforunfolded p_{T,ch jet}<80 GeV/c and p_{T,ch jet}≥^{80 GeV/c,re-} spectively. The triggered sample correspond to a total integrated luminosityof1.6 nb⁻¹.

Inadditionto theonlinetriggers inbothcollision systems,an offlineselectionwasappliedinwhichtheonlinetriggerwas vali- datedandremainingbackgroundeventsfrombeam–gasandelec- tromagneticinteractions were rejected. Toensure ahightracking efficiency for all considered events, the primary vertex was re- quiredto bewithin 10cm fromthecenterof thedetectoralong thebeamaxisandwithin1 cminthetransverseplane[35].

3. Jetreconstructionandbackgroundsubtraction

Jet reconstruction for both the p–Pb and Pb–Pb analysis was performedwiththekT[36]andanti-kT[37]sequentialrecombina- tion jet algorithms as implemented in the FastJet package [38].

The anti-kT algorithm was used for the signal jets while clus- ters reconstructed with the kT algorithm were used to estimate thebackgrounddensityoftheevents.Jetswerereconstructedus- ingchargedtracksdetectedintheTimeProjectionChamber(TPC) [39] and the Inner Tracking System (ITS) [40] which cover the full azimuthal angle and pseudorapidity |

η

|<0.9. Jets were re- constructed usingthe E-schemeto recombinethefour-vectorsof the constituents,assigning the charged-pion massfor each parti- cle. A resolution parameter, R,of 0.4 was used, andthejet area was calculatedbytheFastJetalgorithmusingessentiallyzeromo- mentum particles, called ghosts, witharea 0.005 [41]. Jets were accepted ifthey were fullycontainedin thetracking acceptance:

full azimuth and|

η

jet|<0.5, to guaranteethat the reconstructed jetaxeswereatleastRawayfromtheedgeofthedetectoraccep- tance.

Reconstructedtrackswereacceptediftheirreconstructedtrans- versemomentaexceeded0.15GeV/c,withatleast70spacepoints found in the TPC and at least 80% of the geometrically accessi- blespace-pointsintheTPC. Trackswererequiredtohaveatleast three hits in the ITS used in the fit to ensure good track mo- mentum resolution. To account for the azimuthally non-uniform response ofthetwo innermostlayers oftheITS, theSiliconPixel Detector(SPD),theprimary-vertexpositionwasaddedtothetrack fit,fortrackswithoutSPDpoints,inordertofurtherimprovethe momentumdeterminationofthetrack.Thetrackmomentumreso- lutionwasabout1%at1GeV/c andabout3%at50GeV/c[35].Jets whichcontainedatrackwithpTlargerthan100GeV/c,forwhich thetrackmomentumresolutionexceeded6.5%,wererejected.The tracking efficiency incentral Pb–Pb collisions was 80% for tracks with pT largerthan1GeV/c anddecreased to56%at0.15GeV/c.

In p–Pb collisionsthetrackingefficiencywas 70% fortrackswith pT=⁰.15 GeV/c andincreasedto85%forpT≥^{1 GeV/c.}

Tosuppressthecontributionofjetsconsistingmainlyofback- groundparticles (combinatorial jets),onlyjetscontaining a“hard core” were accepted.Ajet was selected onlyif itoverlappedge- ometrically with a jet reconstructed with only constituents with pT>4 GeV/c. In the kinematic region considered, the hard core selection had similar performance as the selection used in pre- vious works, namely demanding the jet leading track to have a transverse momentum of at least 5 GeV/c [17,18]. PYTHIA pp simulations showedthat applyingsuch a selection was 100% ef- ficientonthejetpopulationforcharged-jettransversemomentum p_{T,ch jet}≥^{25 GeV/c.Thefluctuatingbackgroundin}Pb–Pb collisions affectedthejetenergyscaleincreasingthefull-efficiencythreshold top^sub_{T,ch jet}=^{60 GeV/c(where psub}_{T,ch jet}^isthebackground-subtracted p_{T,ch jet},definedbelowinEq.(6)).Inminimumbiasp–Pb collisions thefragmentationbiasvanishedforp_{T,ch jet}≥^{30 GeV/c.}

3.1. Jet-by-jetbackgroundsubtractioninPb–Pb collisions

Jet measurements inPb–Pb collisions are severely affected by the underlyingevent. Areconstructedjet contains particlesunre- lated tothe hard parton shower.In thisanalysisthe background wassubtractedjet-by-jet.Forthispurpose,meanbackgroundden- sitieswere determined by characterizingevent-by-event thecon- taminationfromsoftparticlesunrelatedtothehardjetsignal.The backgroundtransversemomentumdensity,

ρ

,wasdefinedas

ρ =

median

pT,i

Ai

,

(2)

wherei indicatestheithkT clusterintheevent, pT,i isthetrans- versemomentumoftheclusterandAiisitsarea.ThetwokTclus- ters withhighesttransverse momentum were excluded from the

calculationofthemedian.The average

ρ

inthe10%mostcentral Pb–Pb collisionswas116 GeV/c.Furtherdetailsaregivenin[17].

Totake into account theinfluence of backgroundparticles on thereconstructedjet mass,aquantity m_δ,kcluster

T was evaluatedfor eachkTclusterfollowingtheprocedureoutlinedin[42]

m_δ,kcluster

T

=

j

(

m²_j

+

^p2_T,j^c2

−

^pT,jc

),

(3)

wherethesumrunsover all particlesinside thekT cluster, mj is themassand p_T,j thetransversemomentum ofeach constituent.

Thebackgroundmassdensityisdefinedby

ρ

m

=

^median

m_δ,i

Ai

,

(4)

wherethesubscriptiindicatesagaintheithk_Tclusterintheevent and A_i is the area of the kT cluster. As in the calculation of

ρ

, thetwo leading kT clusterswere excluded fromthe median cal- culation.ForcentralPb–Pb collisions,

ρ

m^{was foundtobe about} 3.6 GeV/c².

Thebackgrounddensities,

ρ

and

ρ

m,wereusedincombination withtwo backgroundsubtractiontechniquesforjetshapeswhich willbedescribedinthefollowing:

(i) Thearea-basedsubtractionmethodcorrectsjet-shape observ- ables forbackground orpileup effectson an event-by-event andjet-by-jetbasis[42].Themethodisvalidforanyjetalgo- rithm andinfrared- and collinear- safe jet shapes. The back- groundischaracterizedby

ρ

and

ρ

m.Ghostsareaddedinthe

η

–

ϕ

planetotheevent,eachofthemmimickingabackground component in a region of area Ag. The shape sensitivity to pileup is determined by considering its derivatives with re- specttothetransversemomentumandmassoftheghostsand extrapolatedby a Taylorseriesto zeropileuporbackground.

Acompletedescriptionofthemethodcanbefoundin[42].

(ii) The constituent subtraction method is a particle-level ap- proachwhichremovesorcorrectsjetconstituents.Theparticle- by-particlesubtractionallowstocorrectboththe4-momentum of the jet andits substructure. Massless ghosts are added to the event such that they cover the

η

–

ϕ

plane. Each jet will therefore contain the real particles and ghosts. A distance measure is defined for each pair of a real particle i and a ghostk:

R_i,k

=

^pT,i

·

(

y_i

−

^y_k^g

)

²

+ ( ϕ

i

− ϕ

_k^g

)

²

,

(5) where yistherapidityand

ϕ

theazimuthalangle.Aniterative background removal procedure starts fromthe particle-ghost pair with smallestdistance. Ateach step thetransverse mo- mentum and mass of each particle and ghost are modified.

The backgrounddensities

ρ

and

ρ

m are used to assign mo- mentum andmass toeach ghost: p_T^g=^Ag

ρ

andm^g_δ =^Ag

ρ

m where Ag istheareaofeach ghost.Ifthetransversemomen- tum ofparticle i islarger thanthe transversemomentum of the ghost,the ghostisdiscarded andthetransverse momen- tum of the ghost is subtracted from the real particle. If the transverse momentum of the ghost is larger than particle i, therealparticleisdiscardedandthetransversemomentumof the ghost iscorrected. The sameprocedure is appliedto the massoftheparticlesandghosts.Allpairsareconsideredand theiterativeprocedureisterminatedwhentheendofthelist ofpairs isreached.Thefour-momentumofthejet isrecalcu- latedwiththesamerecombinationschemeasusedforthejet finding procedure.A completedescriptionofthe methodcan befoundin[43].

The area-based subtraction method was used asthe nominal method for the Pb–Pb analysis to correct the reconstructed jet mass for the influence of backgroundsince it is expected to in- duce zero bias. On the other hand,since track-by-track it is not possibletodeterminewhethera softparticleisbackgroundoran effectoftheinteractionwiththemedium,theconstituentmethod couldpotentiallyremovenon-backgroundparticles.

Thereconstructedtransversemomentumofanti-kTjets,p^raw_{T,ch jet}, iscorrectedaccordingto[44],

p^sub_{T,ch jet}

=

^praw_{T,ch jet}

− ρ _·

A

,

(6)

where A istheareaofthejetand

ρ

isthe p_T-densityofthecon- sideredevent,asdefinedinEq.(2).

3.2. Backgroundinp–Pb collisions

Inp–Pb collisionstheaverage

ρ

and

ρ

mwereabout1.26 GeV/c and0.08 GeV/c²,respectively.Toaccountfortheregionsofthede- tectorwithouteventactivity,anadditionalcorrection[45]wasap- pliedandthehardsignaljetswereexcludedfromthebackground estimate by excludingoverlap ofthekT clusterswithanti-kT jets withp_{T,ch jet}>5 GeV/c.Whiletheoverallbackgroundcontribution is significantly smaller than in Pb–Pb collisions, it was observed thatthewidthofthemassfluctuationscausedbythep–Pb back- groundwas increasedwhen subtractingthe backgroundonajet- by-jetbasiswithrespecttoincludingitintheresponse.Therefore, to minimize thiseffect present in sparse events and to mitigate thedifferentsensitivitiesoftheconsideredsubtractionmethodsto fluctuations,inp–Pb collisionsthebackgroundwasnotsubtracted jet-by-jet(onan event-by-eventbasis),butcorrectedforonaver- ageinthe unfolding,asexplainedin greaterdetail inSec.4.The systematicuncertaintyonthischoicewas assessedbysubtracting thebackgroundindatawiththeconstituentmethodandcorrect- ingonlyforthedetectoreffectsintheresponse(seeSec.6).

4. Jetscaleandresolution

For the Pb–Pb analysis, the jet energy and mass response werestudiedbyembeddingsimulatedppeventsatdetectorlevel, namely including the effects of the detector response, into real Pb–Pb events. The detector response was determined from a PYTHIA6simulation(tuneAwithinitialstateradiationparameter PARP(67)=^{2.5tofittheD0di-jetdata[46])followedbyadetailed} particle transportusing GEANT 3[47] in a detectorconfiguration corresponding to the conditions during Pb–Pb data taking. Prior to embedding the reconstructed tracks from the simulation into Pb–Pb events,an additional p_T-dependenttrackinginefficiency of 2–4%wasappliedinordertoaccountforthelargertrackingineffi- ciencyduetothehighoccupancyforlargeparticledensities[18].

The combinationofPb–Pb andPYTHIAevents willbe referred to as‘hybridevents’.

The same jet reconstruction procedure as in data, see Sec. 3, was appliedto thehybridevents,resultingina sample ofhybrid jets. Thehybridjetswere matchedto theprobejets, whichwere obtained by reconstructing jets from only the PYTHIA events at thedetector level.Notall constituentsofan embedded probejet will necessarily be found in a hybrid jet. In order to relate the hybrid to theprobe jet, a matchingcondition was used.This re- quiredthattheconstituentsofthehybridjetthatcomesfromthe PYTHIAevent mustcarry at least50% of the transversemomen- tumofthePYTHIAjet.Inthecasethatahybridjetwaspairedto twoormoreprobejets,itwasmatchedtotheprobejetwithhigh- est pTandtheotherprobejetswereconsideredlost,reducingthe jet-findingefficiency.Theefficiencyinthe10%mostcentralevents

forchargedjetsincreasedfrom40%atp^sub_{T,ch jet}=^{10 GeV/c to100%}

forp^sub_{T,ch jet}>40 GeV/c.

Region-to-regionfluctuationsofthejetmassandpT-scalewere characterizedby usingthe hybrideventsandcalculatingδpT and δM,definedasthedifferencebetweenthetransversemomentum ormassofthebackground-subtractedhybridjetandtheprobejet [48].Onajet-by-jetbasisalinearcorrelationbetweenδp_TandδM wasobserved.

Thejetmassdistributionsofhybridjetsmatchedtoprobejets within a certain jet pT-interval, showed on average a larger jet mass withrespect to the corresponding spectrum ofthe probes.

Thisoffsetwas duetobackgroundfluctuationsandlimitedpurity andefficiency within a reconstructed jet p_T-interval, resulting in jetmigrationbetweenp_T-intervals.

Detectoreffects onthe jetenergyandmass wereinvestigated bymatchingdetectorleveljetsandparticleleveljetsfromaPythia simulationandcomparingtheir properties(including Pb–Pb back- ground).Thejetswere matchedbasedon distance,inawaythat guarantees a one-to-one match. The constituents of the detector

Fig. 1.(Coloronline)Massresponseusingthearea-basedbackgroundsubtraction methodinthe10%mostcentralPb–Pb collisionsforbackgroundfluctuationsonly (black,shadedhistogram),comparedtothefullresponseincludingdetectoreffects (red,hashedhistogram),foranti-kTjetswithresolutionparameterR=0.4.Msub referstothebackground-subtractedreconstructedjetmasswhileMprobe isthejet massoftheembeddedprobe.Fromtoplefttobottomright,eachpanelrepresenta pT,ch jetregion,40–60,60–80,80–100,100–120 GeVc.

level jetsare all assignedthe pion mass,asis done forthe data analysis, while the particle mass is used for the particle level jet reconstruction. A comparison between the jet mass response dueto backgroundfluctuationsand thefull response,which also contains detector effects, is shown in Fig. 1. While background fluctuationsinduce a positive shiftofthe reconstructedjet mass, detector effects, which are dominated by the finite trackingeffi- ciencyandthemassassumptionofthejetconstituents,reducethe reconstructed jetmass.Thiswas furthercharacterizedby extract- ing the meanand themostprobable value fromthe distribution in Fig. 1,givinga measureof therelative jetmass shift.Therel- ativemass shiftisshowninFig. 2forthe area-basedsubtraction method (left) and the constituent subtraction method (right). In the kinematic range of interest, the mass shift doesnot exhibit a strong dependence on jet momentum. The performance of the constituentsubtractionmethodisslightlybetterthanforthearea- based method sincethe constituent subtractioncorrects partially forthelocalbackgroundfluctuationswhilethearea-basedmethod onlycorrectsfortheaveragebackground.

Since embedding a full PYTHIA event, including the underly- ing event, into the sparse p–Pb event would significantly distort thep–Pb backgroundestimate,theaboveprocedure,devisedwith Pb–Pb collisionsinmind,wasmodifiedforp–Pb collisions.Tomin- imize the distortion, we instead embedded single tracks, whose 4-vectors correspond to jetsreconstructed froma PYTHIA simu- lation(tunePerugia2011[49]) atdetectorlevel,intop–Pb events.

AfterrunningFastJetonthemeasuredeventsincludingembedded PYTHIA tracks, each resulting jet was matched with the particle levelPYTHIAjetassociatedtotheembeddedtrack.

Fig. 3showsthejetmassresolutionforPb–Pb andp–Pb colli- sions asafunctionofthejet massatparticlelevelforprobejets with60<p_{T,ch jet}<80 GeV/c.Astrong dependenceon M_probe is observed.The resolutionforjetswitha smallmassispoorwhile forlargerjet massesitimprovesto25%. Jetswithsmallmassare verycollimatedandtypicallyhaveasmallnumberofconstituents.

The influence of the tracking inefficiency and the contamination of tracks fromthe background on thesejets are large. For large enough pT,ch jet (>40 GeV/c),jetswitha smalljet massare rare and therefore the poor resolution for very collimated jets with smallnumberofconstituentsisnotalimitingfactorinthisanaly- sis,whichwasrestrictedtojetswithp_{T,ch jet}>60 GeV/cforPb–Pb andp–Pb collisions.Forexample,onlyabout16% ofthejetshave amasssmallerthan6GeV/c² withinthe60–80 GeV/c p_{T,ch jet} in- tervalinPYTHIA.

The jet massscale andresolutionin p–Pb collisionsare dom- inated bytracking inefficiency,the massassumptionfor thecon- stituents,and,lessstrongly,bytrackmomentumresolution.Thejet

Fig. 2.Jetmassscalecharacterizedbytherelativemeanandmostprobablevalueoftheresponse.Jetmassscaleisshownasafunctionofprobejet pTforbackground fluctuationsandthefullresponseincludingdetectoreffects,usinganti-kTPYTHIAjetswith R=⁰.4 embeddedintocentralPb–Pb collisions.Left:area-basedsubtraction method.Right:constituentsubtractionmethod.

massresolutioninp–Pb collisionsatsmalljetmassisbyafactor 2betterthaninPb–Pb collisionsduetothemuch smallercontri- butionoftheunderlyingevent.Atlargejet masstheresolutionis similarfor thetwo collision systems,25% for Pb–Pb and 20% for p–Pb,andmainlydrivenbydetectoreffects.

5. Uncorrectedjetmassdistributionsandcorrections 5.1.ComparisonofjetmassinPb–Pb toPYTHIAatdetectorlevel

Itis common useto compare uncorrected Pb–Pb results with embedded pp or PYTHIA events,including in the latter detector and background effects. We perform this comparison and then proceed with the full correction inorder to compare withp–Pb correctedresultsandparticle-leveleventgenerators.

In this section, the jet mass distributions measured in cen- tralPb–Pb collisionsarecomparedtohybriddetector-levelPYTHIA jets. Thebackground was subtracted fromthe jettransverse mo- mentumandmass usingthe area-basedandconstituent subtrac-

Fig. 3.JetmassresolutionasafunctionoftruejetmassM^probe_{ch jet}forprobeswith60<

p^probe_{T,ch jet}(GeV/c)<80GeV/c.Pb–Pb:Anti-kTPYTHIAjetswithR=⁰.4 embeddedinto centralPb–Pb collisionstakinginto accountbackgroundfluctuationsanddetector effects.p–Pb:4-vectorscorrespondingtodetector-levelPYTHIAjetsembeddedinto p–Pb events.

tion methods. A comparison of the jet distributions (normalized per jet) is shown in Fig. 4. It can be observed that the Pb–Pb and PYTHIA distributions are similar, which supports the valid- ity of using embedded PYTHIA for the corrections as discussed inSec. 5.2.The constituentmethodgivessystematicallylower jet mass than the area-based method,due to the different effect of background fluctuations for the two subtraction algorithms, see Sec. 4.The lower panelsofFig. 4 show the ratiobetweenPb–Pb and PYTHIA embedded jets. The ratio asa function of jet mass showsthatthemeasureddistributionsareverysimilartotheem- beddedPYTHIAjets,orpossiblyasmallshifttolowermass,which ishowevermorepronounced fortheconstituentbackgroundsub- traction method. The hint of a shift is more pronounced in the meanjet mass,which isslightlysmallerin Pb–Pb collisionsthan embedded PYTHIAevents,asshowninFig. 5.Alsowhencompar- ingthecorrectedresultswithPYTHIAatparticlelevellaterinthis letter,thedatashowahintofashifttowardssmallermasseswith respecttoPYTHIAwhenconsideringonlystatisticaluncertainties.

Fig. 5.ComparisonofthemeanjetmassinPb–Pb collisionstodetector-levelembed- dedPYTHIAjets,foranti-kTchargedjetswithR=0.4 in10%mostcentralPb–Pb collisions.Backgroundsubtractionwiththearea-basedmethod.

Fig. 4.Detector-leveljetmassdistributionsinPb–Pb dataandPYTHIA(tuneA)embeddedintoPb–Pb collisions.Centrality:0–10%.Anti-kT withR=⁰.4.Left:area-based backgroundsubtraction.Right:constituentbackgroundsubtraction.

5.2. Correctionofjetmasstoparticlelevel

Forthecorrectionofthejetmassmeasurementtoparticlelevel, atwo-dimensionalBayesianunfoldingtechnique[50]fromRooUn- fold[51] was used.Afour-dimensionalresponse matrixwas con- structed withthe following axes: particle-level pT,ch jet, detector- level p_{T,ch jet}, particle-level M_{ch jet} and detector-level M_{ch jet}. For thePb–Pb analysis,detector-leveljetswereobtainedbyembedding detector-levelPYTHIAjetsintoPb–Pb events,runningthejetfinder and applyingthe background subtractionas explained in Sec. 3.

A projectionoftheresponseonthedetectorlevelmass isshown inFig. 4.Asdiscussed inSec.4,theembedded detector-leveljets werematchedtothedetector-leveljetswithoutPb–Pb background.

Thelatterwerematchedtoparticle-leveljetsinsuchawaytoob- tainauniquematchingbetweeneachdetector-levelembeddedjet andthecorrespondingparticle-leveljet.

For the p–Pb analysis, detector-level jets were obtained from embedding detector-level jet four-momentum vectors into p–Pb events(seeSec.4).Thereconstructedembeddedjetswerematched with the particle-level four-momentum vectorscorresponding to thedetectorlevelembedded fourmomenta.Thefour-dimensional matrix contains the smearing in jet pT and mass due to back- groundanddetectoreffects.

The four-dimensionalresponse matrixwas usedto unfoldthe jet pT andmasssimultaneously,takingadvantageoftheobserved strongcorrelationbetweenthejettransversemomentumandmass fluctuations caused by the residual region-to-region background fluctuations,whichreducesoff-diagonalelements intheresponse matrix. The relationship betweenthe transverse momentum and massofthejetatparticlelevelintheresponse,calledtheprior,is obtainedfromPYTHIAsimulations(tuneAforPb–Pb andPerugia 2011forp–Pb).Avariation ofthisassumptionwas consideredin thesystematicuncertainties(Sec.6).

Theunfoldingprocedurewas validatedusingaMCclosuretest by applying the correction procedure to PYTHIA embedded jets.

For the signal and the response matrix, statisticallyindependent datasetswereused.Thebackgroundsubtractedandunfoldedand true distributions agree witheach other to a precision of 5% for p_{T,ch jet}>40 GeV/c. The refolded distribution, obtained by con- voluting the unfolded solution with the response matrix, is in agreement with the measured distribution within the statistical uncertainty.

6. Systematicuncertainties

The systematic uncertainties for the jet mass measurement were determined by varying parameters and algorithmic choices ofthe measurement, corrections fordetectorresponse and back- ground fluctuations. The main systematic uncertainties originate fromtheregularizationoftheunfoldingalgorithm,thebackground subtractionmethodandtheuncertaintyonthedetectorresponse.

ForthePb–Pb analysis,alsothechoiceoftheprior,therelationbe- tweenmassandpT attheparticlelevel,usedintheunfoldinghas animportanteffect.Inthissectionthemethodtoestimatethesys- tematicuncertaintyforeachsourceandtheirmagnitudeincentral Pb–Pb collisionsandp–Pb collisionswillbediscussed.

The unfolding procedure converges after a certain number of iterations. Only relatively small variations in the results are ex- pected when the convergence is reached. The sensitivity to the numberofiterations chosen asdefaultwas estimatedby varying their number over a wide range, where the convergence of the resultis verified. The nominalnumber of iterations used forthe Pb–Pb measurementis6andthenumberofiterationswas varied from3to10.Forp–Pb collisionsthedefaultis3andthenumber was varied between1 and5. Changing the numberof iterations

shifts the full jet mass distribution to higher or lower jet mass, resultingin ananti-correlated shapeuncertainty. Therelativeun- certaintyislargestinthetailsofthejetmassdistributionwhereit amounts to20% inPb–Pb collisions and5–20%inp–Pb collisions fordifferent pT ranges.In thepeak regionof thejet massdistri- butions theuncertaintydoesnot exceed5%(2%) inPb–Pb (p–Pb) collisions.Thesizeoftheuncertaintyinthenumberofiterationsis correlated withthestatistical uncertaintyandtheuncertainty on thedatapointsiscorrelatedpoint-to-point.

The prior used for the Bayesian unfolding was taken from PYTHIA simulations. The mean jet mass as a function of uncor- rectedbutbackground-subtractedjet pT is1–4%smallerinPb–Pb collisions thaninPYTHIAsimulationsasshowninFig. 5.Thesec- ondcentral momentsofthedistributions arestatisticallycompat- ible indicating that the shape of the distribution is unchanged.

Therefore it isreasonable to apply a shift ofatmaximum 4% on the jet massin the prior toestimate a systematicuncertainty to themeasurementduetothepriorchoice.Thisresultsinasystem- aticuncertaintyof10%aroundthejetmasspeak,whichincreases gradually to50%inthetails.Forthep–Pb analysis, asmearingof the massatparticle levelinthe responsematrix wasperformed.

ThenewparticlelevelmassisextractedrandomlyfromaGaussian centeredattheoriginalmasswitha

σ

of2%,roughlycorrespond- ing tothemaximumspreadobservedintheratioofthejet mass distributionintheresponseatdetectorlevelandinthedata.The resultinguncertaintyrangesfrom4%to6%,withthelargestvalue reachedinthefirst pTrange.

For the jet-by-jet background subtraction in Pb–Pb collisions, theresultfromthearea-basedmethodwascomparedtothecon- stituent subtraction. The response matrix forthe methodsis dif- ferentsincethejetmassscalediffersaswas showninFig. 2.The response matrixinboth caseswas obtainedusingtheembedding technique presentedin Sec.4. The systematicuncertainty dueto thebackgroundsubtractionmethodvariesbetween5%atthecen- terofthedistributionand30%inthetails.

As mentionedinSec. 3.2,inp–Pb eventsthe backgroundsub- traction introduces additional fluctuations due to the region-to- regionfluctuationsofthebackground,whichleadstoabroadening ofthejet massdistributionaftersubtraction.Itwas thereforede- cidednottoperformthesubtractionevent-by-eventandjet-by-jet, and instead include the background in the response matrix and correct for in it the unfolding. As an extreme variation for the systematic uncertainty, the background was subtracted event-by- eventinthedatawiththeconstituentmethod,whichislesssen- sitivetofluctuationsthantheareamethod,andcorrectedonlyfor detectoreffectsusingthePYTHIAresponse.Thejetmassdistribu- tionscorrectedwiththetwoassumptionsdifferby5%inthepeak regionandthedifferenceincreasesgraduallyupto40%inthelow- masstail.Thesevariationsweretakenassystematicuncertainties.

The uncertainty in the detector response was dominated by theuncertaintyinthetrackingefficiency,whichwasestimatedby varyingtrackqualitycutsandfoundtobe3–4%.Thetrackingeffi- ciencyinthedetectorsimulationwasvariedaccordingly,providing analternativeresponsematrixwithwhichtorepeattheunfolding.

Observeddifferenceswithrespecttothenominalresultvaryfrom 10% to 40%and5% to 30%in Pb–Pb and p–Pb,respectively,with thelargestuncertaintyinthetailsofthedistributions.

Allsystematicuncertaintieswereaddedinquadratureforeach M_{ch jet} bin.Theuncertaintiesaffecttheshapeofthejetmassdis- tribution and the normalization applied causes long-range anti- correlations. The uncertainty on the mean jet mass as a func- tion of p_{T,ch jet} was evaluated onthe unfolded distribution using the variations mentioned above and shown in Table 1. The to- tal systematic uncertainty in the mean jet mass increases from 6% for jets with 60< p_{T,ch jet}<80 GeV/c to 9.0% for jets with

Table 1

Systematicuncertaintyinmeanjetmassfromdifferentsourcesinthe10%mostcentralPb–Pb collisions(left)andminimum-biasp–Pb collisions(right).

SourcepT,ch jet (GeV/c)

Pb–Pb p–Pb

60–80 80–100 100–120 60–80 80–100 100–120

Prior 1.0% 3.0% 5.0% 0 0 0

Background 3.0% 3.0% 5.0% 1.0% 0.5% 1.0%

Tracking efficiency 5.0% 5.0% 5.0% 3.0% 3.0% 3.0%

Unfolding (iterations, range) 1.0% 3.0% 4.0% 0.5% 1.0% 4.0%

Total 6.0% 8.0% 9.0% 3.5% 3.5% 4.5%

Fig. 6.Fully-correctedjetmassdistributionforanti-kT jetswith R=⁰.4 inp–Pb collisions,comparedtoPYTHIAandHERWIGsimulationsfor threerangesof pT,ch jet. Statisticaluncertaintiesindataaresmallerthanthemarkersandinthemodelsaresmallerthanthelinewidth.

Fig. 7.Fully-corrected jet mass distribution for anti-kTjets withR=0.4 in minimum bias p–Pb collisions compared to central Pb–Pb collisions for three ranges ofpT,ch jet. 100<p_{T,ch jet}<120 GeV/cinPb–Pb centralcollisions.Thesystem-

aticuncertaintyinp–Pb collisionsisabouttwotimessmallerthan in central Pb–Pb collisions due to the much smaller underlying eventcontribution.

7.Resultsanddiscussion

7.1.JetmassmeasurementsinPb–Pb andp–Pb collisions

Thefullyunfolded jet massdistributions includingallsystem- aticuncertainties,measuredinp–Pb collisionsat√

sNN=⁵.02 TeV inthreeranges of pT,ch jet between60and120 GeV/c are shown in Fig. 6 and compared with PYTHIA Perugia 2011 and HER- WIGEE5C[30,52]. Minimum-biastriggered events were used for p_{T,ch jet}<80 GeV/c, while the online jet triggered event sam- plewas usedfor p_{T,ch jet}≥^{80 GeV/c. The agreementof data and} PYTHIAis within 10–20% for mostof the M_{ch jet} range. The de- viationsincrease for the low and highmass tail andcan exceed 30–50% for the intermediate pT,ch jet range. The agreement with HERWIGisslightlyworse,mostly inthelowmasstailofthedis- tributionandinthehighestpT,ch jet interval.Consideringthegood agreementwithsimulations andthatthejet nuclearmodification factors RpPb and QpPb measurements show nocold nuclearmat- tereffects[45,53–55],thep–Pb measurement(andPYTHIA)canbe

usedasa referencefortheassessmentof thehot nuclearmatter effectsinPb–Pb collisions.

Fig. 7 showsthe comparisonofthejet massdistribution,nor- malizedperjet,incentralPb–Pb collisionsat√

sNN=².76 TeV and the p–Pb collision measurement. It can be observed that the jet mass distribution in Pb–Pb collisionsis shifted to smaller values withrespecttothemeasurementinp–Pb collisions forp_{T,ch jet}<

100GeV/c.

Fig. 8 shows the ratio between the jet mass distribution in the 10% most central Pb–Pb collisions and p–Pb collisions. The systematic uncertainties are propagated into the ratio as uncor- related. The center-of-massenergyatwhich thePb–Pb and p–Pb collisions weretakenisdifferent, √

s_NN=².76 TeV for Pb–Pb and

√sNN=⁵.02 TeV forp–Pb collisions.Thisisexpectedtointroduce a smalldifferenceinthe jetmassdistributions duetoa different shapeintheunderlyingjet pT-spectrumandadifferentquark-to- gluon ratio.Therefore,the figureshows alsothesame ratiofrom particlelevel simulatedPYTHIApp collisions (tunePerugia 2011) at the two energies. Considering statistical uncertainties only in the ratio, a shift to lower jet masses in Pb–Pb is observed for p_{T,ch jet}<100 GeV/c,consistentwiththePYTHIAembeddedresults inSec.5.1.Includingthesystematicuncertaintiesinourmeasure- ments,thedecreasingtrendoftheratioasafunctionofM_{ch jet} is

Fig. 8.Ratiobetweenfully-correctedjetmassdistributionforanti-kTjetswithR=⁰.4 incentralPb–Pb collisionsandminimumbiasp–Pb collisions.Theratioiscompared totheratioofmassdistributionsofPYTHIA(tunePerugia2011)at√

s=².76 TeV and√

s=⁵.02 TeV (widthofthebandrepresentsthestatisticaluncertainties).

Fig. 9.Fully-correctedmeanjetmassasafunctionofpT,ch jetforanti-kTjetswith R=⁰.4 inminimumbiasp–Pb collisionsat√

sNN=⁵.02 TeV comparedtocentral Pb–Pb collisionsat√

sNN=2.76 TeV.

compatiblebetweendataandPYTHIAandnosignificantreduction injetmassinPb–Pb collisionsisobserved.

Thecomparisonof thejet massinPb–Pb collisions relativeto p–Pb collisions isfurther established by presenting themean jet massasafunctionofp_{T,ch jet}inFig. 9.Thedifferenceinthemean jetmass forthe twocollision energiesconsidered isbetween0.2 and 0.5 GeV/c² in the PYTHIA simulation.This difference in the mean jet mass isindicated by a filled box attachedto the p–Pb datapoints inFig. 9.Forthe lowest pT,ch jet rangeinPb–Pb colli- sionsthemeanjetmassexhibitsareductionwithrespecttop–Pb measurements,limitedtoaboutonestandarddeviation.Forhigher pT,ch jet themeanjetmassinthetwosystemsiscompatiblewithin systematicuncertainties.

7.2. Modelcomparisonanddiscussion

The jet mass measurements for central Pb–Pb collisions for three p_{T,ch jet} intervalscompared to severalevent generators are shownin Fig. 10. PYTHIArepresents the expectationwithout jet quenchingwhileJEWEL[56,57]andQ-PYTHIA[58](withPQMge- ometry [59]) are two models withmedium-induced energyloss.

InJEWEL each scatteringofthe leading partonwithconstituents from the medium is computed giving a microscopic description of the transport coefficient, ^q.ˆ ^{By default, JEWEL does not keep} track ofthe momenta of the recoiling scattering centers (“recoil off”). This leads to a net loss of energy and momentum out of thedi-jetsystem,andisexpectedtomostlyaffectlow-p_T-particle production.Forthe jet massmeasurement, low-momentum frag- mentsareimportant,soJEWELwasalsoruninthemodeinwhich itkeepstrackofthescatteringcenters(“recoilon”).Inthatmode,

more softparticlesare generated,some ofwhichhavevery large angles with the jet and will contribute to the background esti- mateintheevent.TheJEWELauthorsimplementedabackground subtraction infull jetsby introducing “fake” neutral constituents used for the 4-momentum subtraction. Since the pp charged jet mass distribution isreproduced by shifting thefull jet mass dis- tributiontowardslowermasses,theJEWELbackground-subtracted chargedjetmassisobtainedbyshiftingthebackground-subtracted full jet mass. Q-PYTHIA modifies the splitting functions in the PYTHIAeventgenerator,resultinginmedium-inducedgluonradia- tionfollowingthemultiplesoftscatteringapproximation.Both jet quenchingmodelsreproducethesuppressionobservedininclusive high-pTparticleandjetproduction[57,58].

ThejetmassisstronglyoverestimatedbyQ-PYTHIAduetothe strongbroadeningofthejetprofileclosetothejetaxis.AlsoJEWEL with “recoil on” significantly overestimates the jet mass. JEWEL

“recoil off”underestimates the jetmass duetothe large amount of out-of-cone radiation,which doesnot hadronize in thismode of the generator. The vacuum expectation from PYTHIA, while slightlyoverestimating the jet massforlower p_{T,ch jet} whencon- sideringstatisticaluncertaintiesonly,iscompatiblewiththePb–Pb measurementwithinsystematicuncertainties.ThePb–Pb meanjet massasafunctionofp_{T,ch jet}iscomparedtotheeventgenerators inFig. 11.Thelinearincreaseofthemeanjetmasswithjet pT is expectedfromNLOpQCDcalculations[60].

Previous jet shape and jet fragmentation function measure- ments clearlyfavorJEWELwith“recoilon”overQ-PYTHIA[20,19, 61–64]. Despite the difference in the fragment distributions be- tween Q-PYTHIA and JEWELwith “recoilon”, Fig. 10shows that bothmodelspredictasimilarlargeincreaseofthejetmass,which is excluded by the measurement. JEWEL “recoil off”,which does not describetheprevious measurementswell becauseitdoesnot includeallsoftradiation,givesabetterdescriptionofthejetmass thanJEWEL“recoilon”.Thedifferencebetweenthejetmassdistri- butionsinJEWELwith“recoilon”an“recoiloff”indicatesthatthe jetmassissensitivetothesoftfragmentsatlargeanglewhichare producedbyhadronisationofrecoilpartonsintheJEWELmodel.

7.3. Summary

The first jet mass measurement in heavy-ion collisions for charged jets (60<p_{T,ch jet}<120 GeV/c) was reported andcom- paredtop–Pb referencemeasurementsandmodelswithandwith- outquenching.Thepresentedresultsarethefirstattempttoaccess thevirtualityevolutionofthehardpartonsinheavy-ioncollisions.

By constraining both energyand virtualityexperimentally, differ- ential jet mass measurements could provide further non-trivial testsformodelsofin-mediumshowerevolution.

Fig. 10.Fully-correctedjetmassdistributionfor anti-kT jetswith R=0.4 inthe 10%mostcentralPb–Pb collisions comparedtoPYTHIA withtunePerugia2011and predictionsfromthejetquenchingeventgenerators(JEWELandQ-PYTHIA).Statisticaluncertaintiesarenotshownforthemodelcalculations.

Fig. 11.Fully-correctedmeanjetmasscomparedtoPYTHIAPerugia2011andthejet quenchingeventgenerators(JEWELandQ-PYTHIA)foranti-kTjetswithR=⁰.4 in the10%mostcentralPb–Pb collisions.

TheratioofthejetmassdistributionincentralPb–Pb collisions andminimum-biasp–Pb collisionsiscomparedtothatinPYTHIA Perugia2011simulationsatthetwo center-of-massenergies. The dataratio iscompatiblewith thePYTHIA expectationatthe two center-of-massenergies withinsystematicuncertainties. Ahintof a difference within statisticaluncertainties only in the ratioand inthemeanjet massinthelowest p_{T,ch jet} rangeisofinterestto motivatefurtherworkonreducing thesystematicuncertaintiesin ordertoincrease theprecision injetmassmeasurements aswell as pursuing more differential studies, for example with respect to hard fragmenting jets. The fully-corrected results are consis- tentwiththeobservationbasedondetectorlevelcomparisonwith PYTHIAembeddedjets.ThemeasuredjetmassinPb–Pb collisions isnotreproducedbythequenchingmodelsconsideredinthislet- terandisfoundtobeconsistentwithPYTHIAvacuumexpectations withinsystematicuncertainties.Theseresultsarequalitativelycon- sistentwithpreviousmeasurements ofjetshapesattheLHC[20, 62],whichshowonlyrelatively smallchangesoftheparticledis- tributionsinjetsinPb–Pb collisionscomparedtoppcollisions.The JEWELmodelwith“recoil on”,whichdescribestheexisting mea- surementsoffragmentdistributionsinjets[19,20]reasonablywell [61,63],predicts asignificantincrease ofthejet mass,contraryto whatisobservedinthemeasurement.

The observed suppression of jet yields in the presence of a dense medium, RAA<1 [65], is interpreted as due to radiated partons lost or scattered out of the jet cone. Therefore, one re- constructsasubset oftheentirepartonshower within ajet with resolutionparameter0.4.Intheextremecasethatonlytheleading partonwereto escapethe medium,andthen showerin vacuum, onewouldreconstructthemassoftheleadingpartonatthepoint

ofexit.Since alsothevirtualityevolutionofthepartonshower is modified in the presence of jet quenching, one would expect in suchascenariothattheescaping(reconstructed)jetsexhibitare- ducedjet mass withrespectto thepp andp–Pb references [21].

The data show that the jet mass is consistent within uncertain- tiesin Pb–Pb and p–Pb collisions within a fixed pT,ch jet-interval, implyingthatthesoftradiationoutsidethejetconedoesnotsig- nificantlyaltertherelationbetweenpTandthemassoftheparton.

Acknowledgements

The ALICE Collaboration would like to thank all its engineers andtechniciansfortheir invaluablecontributions totheconstruc- tion of the experiment and the CERN accelerator teams for the outstanding performance ofthe LHC complex.The ALICECollab- oration gratefully acknowledges the resources and support pro- videdbyallGridcentresandtheWorldwideLHCComputingGrid (WLCG) collaboration. The ALICE Collaboration acknowledges the followingfunding agenciesfortheir supportinbuildingandrun- ningtheALICEdetector:A.I.AlikhanyanNationalScienceLabora- tory(YerevanPhysicsInstitute)Foundation(ANSL),State Commit- teeofScienceandWorldFederationofScientists(WFS),Armenia;

Austrian AcademyofSciences andNationalstiftungfür Forschung, Technologie und Entwicklung, Austria; Ministry of Communica- tions and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Financiadora de Estudos e Projetos (Finep) and Fun- dação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Science & Technology of China (MSTC), Na- tional Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOEC), China; Ministry of Science, Edu- cation and Sportand Croatian Science Foundation, Croatia; Min- istryofEducation,YouthandSportsoftheCzech Republic,Czech Republic; The Danish Council for Independent Research | Natu- ral Sciences, the Carlsberg Foundation and Danish National Re- search Foundation (DNRF), Denmark; HelsinkiInstitute of Physics (HIP),Finland;Commissariatàl’EnergieAtomique(CEA)andInsti- tut National de Physique Nucléaire et de Physique des Particules (IN2P3) andCentre Nationalde la Recherche Scientifique(CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung undTechnologie (BMBF)andGSI Helmholtzzentrum für Schweri- onenforschung GmbH, Germany; Ministry of Education, Research andReligiousAffairs,Greece;NationalResearch,Developmentand Innovation Office, Hungary; Department of Atomic Energy Gov- ernment of India (DAE) and Council of Scientific and Industrial Research (CSIR), New Delhi, India; Indonesian Institute of Sci- ence, Indonesia;CentroFermi–MuseoStoricodellaFisica eCen- tro Studi e RicercheEnrico FermiandIstituto Nazionale diFisica